The invention relates to a process and apparatus for introducing heat into contaminated soil to vaporize and remove volatile contaminants. The process utilizes spaced apart electrodes penetrating the soil. Electric current is passed between the electrodes to heat the soil and the contaminants. The vaporized contaminants are recovered from the soil through vapor extraction wells.
Contaminated soils are a matter of national concern. Many contaminants have the ability to flow into aquifer systems, thereby contaminating the public water supply. The depth at which some contaminants occur renders the use of excavation prohibitively expensive. It is therefore desirable to have a method that can effectively remove the contaminants in-situ, where depth is not a factor.
It is known to use electricity to heat soil. This has been practiced in connection with heating subterranean heavy oil reservoirs, to reduce the viscosity of the oil so it can be recovered. It has also been used in the environmental field to heat contaminated soil To vaporize contaminants, which are subsequently extracted from the soil. The present invention is concerned with this latter application of electrical heating.
In general, the known scheme used involves:
providing spaced apart electrode wells having tubular electrodes penetrating the soil to be heated;
causing current to pass through the soil from one electrode to the otherxe2x80x94the current moves through conductive connate or indigenous moisture in the soil and heats the resistive soil at the same time, with the result that volatile contaminants in the soil vaporize; and
providing vacuum or suction extraction wells between the electrode wells; and
applying suction to some or all of the extraction wells, to extract the vaporized contaminants from the soil.
In these operations, it is common to use current directly from the local power system, which is normally delivered at 60 Hz.
In greater detail, heat enhances the remediation of contaminated soils in-situ. It does this by increasing the vapor pressure of volatile organic compounds and semi-volatile organic compounds, increasing the solubility of contaminants in the water phase, accelerating the growth rate of bacteria involved in bio-remediation strategies, and decreasing the time required to achieve natural attenuation (natural degradation process of contaminants within the soil).
FIG. 1 shows the vapour pressure relationship for benzene (C6H6), a component often involved in contaminated soils. The curve represents the phase boundary of benzene. Above the curve, benzene naturally exists in liquid phase; below the curve it exists in the gas phase. An increase in temperature from standard conditions of 15xc2x0 C. to 80xc2x0 C. changes the phase of benzene from liquid to gas at atmospheric pressure. The average pressure in the soil can be reduced to one third of an atmosphere as a result of vacuum applied at the extraction wells. As indicated in FIG. 1, at this pressure the temperature needs only to exceed 50xc2x0 C. for benzene to go into the gas phase. Once the contaminant is in the gas phase, it can be recovered from the soil at the extraction wells.
As mentioned, methods have been proposed that use electrical energy for heating in combination with suction extraction through xe2x80x9cwellsxe2x80x9d to remove volatile organic compounds from the contaminated soil. For electrical heating to be effective, the heat distribution through the contaminated soil should be uniform and achieved in as short a time as possible. The present process and apparatus incorporate features which enhance efficient and uniform soil heating.
The present invention provides process and apparatus for electrically heating contaminated soil. This may be done simply to raise the temperature of the soil, for example to mobilize contained viscous oil so that it may drain into a recovery pit. Preferably it is done so as to raise the temperature of soil and contaminants in the soil sufficiently, so that volatile contaminants are vaporized and can be removed from the soil.
One preferred and specific embodiment of the invention comprises:
Providing spaced apart, tubular electrode wells having electrodes penetrating the contaminated soil. The electrode wells are arranged in a pattern (for example, parallel spaced rows) which defines a region or volume of contaminated soil to be heated;
Providing tubular perforated extraction wells penetrating the defined soil region. These extraction wells are connected with means, such as a vacuum pump, for applying suction to remove vapors and/or liquids from the soil region. The extraction wells are located between the electrode/injection wells and create low pressure sinks;
Each electrode comprises a length of conductive pipe having apertures or openings extending through the pipe side wall over intervals at the top, middle and bottom. Each electrode well further comprises means for conveying pumped water from ground surface into the electrode and injecting it under pressure into the contaminated soil through the top and bottom openings. The electrode well further has return conduit means for returning water to ground surface from the middle openings. By means of this arrangement, one part of the water introduced through the electrode can be circulated along the electrode and returned through the middle openings, thereby cooling the electrode and the soil at the electrode ends, where current density is concentrated. In addition, another part of the water can be injected and moved radially out into the region of soil to conduct heat convectively toward the extraction wells;
The proportions of water circulated to cool the electrode and adjacent soil at each end of the electrode and water injected outwardly into the contaminated soil, can be controlled by throttling the return conduit means;
Multi-phase alternating current is applied to the electrodes to induce flow of current between electrodes, thereby heating contaminated soil; and
Suction is applied to the extraction wells to recover contaminants in vapor and/or liquid form.
One broad process aspect of this embodiment of the invention therefore comprises: electrically heating a region of soil bound or defined by spaced electrodes, by passing current between the electrodes; and injecting water under pressure into the soil at the electrodes to transfer heat by convection out into the soil region and toward the extraction wells. Preferably, water injection is conducted simultaneously.
Preferably, heating is conducted by this process of coupling electrical heating with heat transfer by convection to raise the temperature of the soil region sufficiently to vaporize contained contaminants. These vaporized contaminants are then removed through extraction wells positioned between the electrodes.
This process aspect addresses the following problem. The power density in watts per cubic meter decreases from a single electrode as the inverse of the radius raised to the second power. Otherwise stated, electrical heating of soil decreases dramatically as the distance from the electrode increases. Water, injected at the electrode, can function to absorb heat adjacent the electrode and carry it out into the further reaches of the soil region, where it is transferred to the cool soil. In this way, the rate of increasing the temperature of the soil region is improved.
Another preferred feature of the embodiment previously described is that the circulation of cooling water is initiated through the soil at the ends of the electrode, where current density is concentrated. If this is not done, there is a possibility that moisture in the soil at the electrode ends will evaporate, reducing the conductivity in those zones. By cooling these zones with water, this problem is mitigated and current can more uniformly flow from the entire electrode and more power can be input to the electrode.
In another broad aspect of the previously described preferred embodiment;
water is injected into the soil through the openings at each end of the electrode and part of it is returned to ground surface through the middle openings, thereby cooling both the electrode and the adjacent soil at the ends of the electrode;
thermocouples can be mounted at each end of the electrodes to monitor temperature; and
throttling of the water return conduit can be applied in response to the measured temperatures, to ensure that cooling is adequate.
In an apparatus aspect of the previously described embodiment, there is provided an electrode well for penetrating into contaminated soil from ground surface and enabling simultaneous introduction of electrical current into the soil, injection of water into the soil and return of externally circulated cooling water, comprising a tubular conductive electrode having a side wall forming short intervals of openings at the top, middle and bottom of the electrode, the electrode having an internal bore comprising top, middle and bottom portions; first conduit means, extending into the top portion of the bore, for supplying water to the top openings; second conduit means, extending into the lower portion of the bore, for supplying water to the bottom openings; third conduit means, extending into the middle portion of the bore, for returning water, entering through the middle openings to ground surface; means for sealing the bore top portion from the middle portion; means for sealing the bore middle portion from the bottom portion; and means for delivering power to the electrode.
In a preferred apparatus feature, the electrode is provided with electrically isolated end caps. The function of the caps is to ensure that current flow from the ends is minimized and that the current is directed radially from the electrode into the contaminated soil. If the end caps are not electrically isolated, current would preferentially flow from the ends into regions or zones that could not be effectively cooled, resulting in hot spots above and below the electrode. As the region adjacent each electrode end increases in temperature, it becomes less and less resistive. Consequently, more of the current, taking the path of least resistance, will flow from the ends of the electrode and not radially into the contaminated soil where heating should occur. The provision of electrically isolated end caps alleviates this problem.
In another preferred aspect of the invention, the voltage and phase distribution between electrodes are controlled and varied so that electrical current is forced to flow as uniformly as possible through the soil region, even though the soil region is characterized by heterogeneous electrical properties. This feature is most advantageously used with vertical stacking of two or more electrodes, as described below.
In another preferred aspect, the quantity of power to each electrode is controlled. As a result, increased power can be delivered to the contaminated soil during heating operations using a method herein referred to as Time Distributed Power Control. This involves adjusting the input power to an electrode by varying the number of cycles of voltage applied to the electrode over an interval of time. Time Distributed Power Control (xe2x80x9cTDCxe2x80x9d) is used in combination with Inter-Phase Synchronization. Inter-Phase Synchronization (xe2x80x9cIPSxe2x80x9d) involves the application of specific phases of 3-phase AC power to specific electrodes over an interval of time and then changing the phases applied to the electrodes over subsequent intervals of time. When TDC and IPS are used in combination, more power can be inputted into the contaminated soil as it is possible to have up to all of the electrodes conducting current at any given moment. This process can, subsequently, heat the soil faster and more efficiently.
In another preferred aspect, the electrode wells and extraction wells are arranged in a pattern that is adaptable to the shape of the contaminated soil deposit. This pattern has the flexibility to increase the number of electrodes in the pattern with as few as a single electrode. Also the positions of the electrode and extraction wells enable creating pressure distribution in the soil, with an array of pressure sinks at the extraction wells and pressure sources at the electrode wells. This encourages effective recovery of contaminants and facilitates uniform heat transfer by convection. More particularly, the pattern involves placing the electrodes in a row and column configuration with the electrodes spaced a sufficient distance apart to enclose the region of soil to be treated. Different phases of power are applied to different electrodes to cause current to flow from any one electrode to its adjacent electrodes, uniformly heating the region of soil. By introducing TDC into the process, the power applied to an electrode may be individually controlled. When using TDC in combination with IPS, the phases of power applied to individual electrodes may be alternated to re-orient the flow of current among the electrodes as required to uniformly heat the entire region of contaminated soil.